Gas discharge lamps, in particular fluorescent lamps, are used for illumination purposes in many ways. An exemplary use for gas discharge lamps is the background illumination (backlight) of display units that are not self-illuminating, such as liquid crystal displays (LCD""s).
Gas discharge lamps include a lamp body, the shape of which depends on the application, in which a gas discharge chamber is implemented between two electrodes. The chamber is filled with a gaseous atmosphere made of a noble gas or a mixture of noble gases, such as argon and xenon, and at least a slight admixture of mercury. The noble gases are necessary for implementing the gas discharge, but contribute only slightly to generating light. The mercury atoms, in contrast, are excited by collisions with free electrons to emit ultraviolet light, which, in the case of fluorescent lamps, is converted into visible light by a fluorescent layer on the inside of the lamp. As the temperature rises, the mercury vapor pressure, and therefore the number of mercury atoms available in the gas atmosphere to produce light, increases. However, since the mercury atoms in the gas atmosphere also have a light-absorbing effect, an optimum operating temperature of, for example 50xc2x0 C., results in regard to the light yield of the gas discharge lamp. If the temperature rises above, or drops below, the optimum operating level, the light yield diminishes.
Positioning a thermoelectric cooler at an arbitrary position on the outside of the gas discharge lamp is known, for example, from U.S. Pat. No. 5,909,085. Since, in principle, mercury condenses at the coldest point in the gas discharge chamber, the mercury vapor pressure may be kept constant by activating the thermoelectric cooler at lamp temperatures above 50xc2x0 C., without requiring cooling of the entire gas discharge lamp.
In order to reach and/or maintain the optimum operating temperature as rapidly as possible upon starting the gas discharge lamp, or when operating at low ambient temperatures, a heating spiral can additionally be wound around the known gas discharge lamp over the entire length of the lamp. Relatively large leakage capacitances between the gas discharge lamp and the heating spiral result from this. Due to these leakage capacitances, operation of the gas discharge lamp at high frequencies, above 10 kHz, leads to corresponding output losses. On the other hand, however, high-frequency activation of gas discharge lamps is desirable, due to the higher associated light yield, the gas column burning more stably without going out in the current zeros, and the phase shift between the lamp current and the lamp voltage approaching zero.
As already mentioned, gas discharge lamps may have a lamp body having a unique shape depending on the application. An illumination unit implemented as a back-lit backlight is known from WO 98/12471, in which essentially U-shaped fluorescent lamps are arranged in a metallized light box, which is open on one side and covered with a diffuser to promote uniform light emission.
Objects of the present invention are to achieve operation of gas discharge lamps with as little leakage as possible and as large a light yield as possible, in a simple manner.
According to one formulation of the present invention, the above and other objects are achieved by an illumination unit having at least one essentially U-shaped gas discharge lamp, which contains mercury for gas discharge and whose electrodes are connected to output terminals of a high-frequency driver circuit. The output terminals of the high-frequency driver circuit are each electrically floating and the gas discharge lamp is capacitively coupled to an electrical ground in its central region. Accordingly, as will be described in more detail below, the lamp current is drastically reduced in the central region of the gas discharge lamp, where the parallel lengthwise parts of the gas discharge lamp are connected to one another. Heat generation is, accordingly, drastically reduced at this location and the coldest point in the gas discharge chamber, a point at which the mercury may condense, forms. In this way, effective regulation of the mercury vapor pressure in the gas discharge lamp is achieved in a simple manner, without requiring active cooling means with independent current consumption.
Capacitive coupling of the central region of the gas discharge lamp to electrical ground is equivalent to a short circuit at this region. Thus, the lamp current in the parallel lengthwise parts of the gas discharge lamp is not reduced in any way, but rather is increased. Due to very slight leakage capacitances, which cannot be completely avoided, between the two parallel lengthwise parts of the gas discharge lamp and electrical ground, the lamp current in each of the two lengthwise parts slightly decreases equally in the direction extending from the respective electrode up to the central region of the gas discharge lamp. The leakage field resulting from the lamp current in the two parallel lengthwise parts, therefore, totals zero.
In order to be able to effectively dissipate excess heat in the central region of the gas discharge lamp, the gas discharge lamp may be additionally coupled to a thermal ground, the thermal and electrical grounds can be formed in practice by a single component, for example, a plate. In this case, the thermal coupling may be improved further with the aid of heat conduction paste. However, in any case, the quantity of heat to be dissipated is much lower than in the known gas discharge lamps, because the heat generation in the central region of the lamp is significantly reduced in the gas discharge lamp according to the present invention.
In a preferred embodiment of the illumination unit according to the present invention, the electrical and/or thermal ground comprises a metallic light box. Within the metallic box at least one gas discharge lamp is arranged in a way that the electrodes of the gas discharge lamp project out of one side of the light box and the central region of the gas discharge lamp presses against the opposite side of the light box.
In order to make the output terminals of the high-frequency driver circuit electrically floating, this circuit preferably has an output transformer having windings respectively on the circuit and lamp sides, the output terminals being implemented at the ends of the winding on the lamp side.